Inappropriate cell proliferation occurs in many pathological settings including cancer and atherosclerosis. The mammalian cell cycle is regulated by a network of signaling pathways that ensure cell division occurs with high fidelity and only under appropriate conditions. Many of these pathways converge on cell cycle check points that control passage from one stage to the next. Check points control the transition of cells from a quiescent state back into the cell cycle and the transition from G1 into S phase. Two proteins required for these transitions are the retinoblastoma protein, which controls the expression of genes required for G1 progression and the cell division cycle 6 protein (Cdc6), which is required for initiating DNA replication at the G1/S transition. The level and activities of these proteins are regulated by the concerted actions of cell cycle regulated protein kinases and protein phosphatases. Recent evidence has shown that a calcium-regulated subunit of protein phosphatase 2A (PP2A) is involved in dephosphorylation of both of these proteins. This subunit (PR70) directly interacts with the retinoblastoma protein and Cdc6, and induces their dephosphorylation by targeting the catalytic components of PP2A to these substrates. The regulation of the PR70-containing form of PP2A by calcium has led to proposal of a novel signaling mechanism that allows cross-talk between agents that modify calcium signaling and agents that induce cell proliferation. The goals of this proposal are to test the hypothesis that the PR70 subunit of PP2A mediates calcium regulation of G1 progression by controlling the dephosphorylation of the retinoblastoma protein and Cdc6.